#include <Eigen/Core>
#include <iostream>
using namespace Eigen;

int main() {
  MatrixXd m(2, 5);
  m.resize(4, 3);
  std::cout << "The matrix m is of size " << m.rows() << "x" << m.cols()
            << std::endl;
  std::cout << "It has " << m.size() << " coefficients" << std::endl;
  VectorXd v(2);
  v.resize(5);
  std::cout << "The vector v is of size " << v.size() << std::endl;
  std::cout << "As a matrix, v is of size " << v.rows() << "x" << v.cols()
            << std::endl;

  Matrix4d mm;    // 固定大小
  // mm.resize(1,5); // 失败
  mm.resize(4,4); // 合法的
  return 0;
}